To survey military or other security-related operations and activities in space, the object of which can be to disturb or destroy satellites, in order to enable the detection of any anti-satellite spacecraft or other space activities, it is necessary to have a situational awareness of space close to the earth. Such operations can also only serve for reconnaissance purposes without initially causing any direct damage. At present it is normal to use ground-based radar systems or telescopes for this purpose. Depending on the flight path of a satellite, it can, however, take days until this satellite reaches the field of view of such a detection instrument. Using optical telescopes, there can also be other external circumstances, such as darkness or poor weather, which prevent or at least substantially impair detection.
Furthermore, an early warning system for the detection of ballistic missiles using satellites is known: One example is the US Defence Support Program DSP, which uses satellites in the geostationary orbit. As part of the US follow-up program, the Space Based Infrared System SBIRS, as well geostationary (SBIRS-High) as also low-flying (SBIRS-Low) satellites were used. The SBIRS-Low Project has, however, never gone beyond the concept and experimental phase because of the expense and costs (global system). This kind of early warning serves as a detection of incoming missiles and serves as an early release of an alert and a pre-setting of radar equipments of a defensive architecture.
Detection can be broken down into two different tasks, i.e., detection of one or more incoming missiles in the boost or combustion phase and tracking of the burnt-out missiles in the succeeding free-flight phase or free-flight trajectory.
Early warning satellites in geostationary orbit offer advantageous fixed geometric conditions. This means that a small number of satellites is possible. A particular disadvantage in this case is that due to the great distance of the satellite from the target area or target object the requirements regarding the detection sensor system increase. Furthermore, large space transport carriers are necessary for positioning the satellites.
Due to the geometric conditions, a geostationary satellite has to be able to detect an incoming missile from a great distance and against the natural radiation background of the earth and also distinguish it from civil carriers or other heat sources. Tracking in the free-flight phase using geostationary satellites is not considered technically feasible.
Satellites in high elliptical orbits are similarly complex and expensive like GEO systems (geostationary satellites) but require an increased number of satellites compared to these.
LEO satellites have a comparatively short distance to the object under surveillance, which enables small cost-effective satellites to be constructed. A disadvantage, however, is the small surveillance or viewing area and field of view, respectively, of a LEO satellite, which due to orbit mechanics cannot be fixed on an area. Previous concepts typically require more than 20 or even more than 30 satellites for a global operational capability, such as, for example, described in DE 198 45 911 A1. The number of satellites required could not be significantly reduced by these concepts even if limited to an area of interest, so that a global surveillance capability was automatically provided when designing a satellite constellation.